Exemplary embodiments of the invention described herein relate generally to measurement of electrical conductivity of fluids. More particularly, exemplary embodiments of the invention described herein relate to calibration of digital conductivity measuring systems.
Electrical conductivity measurements are commonly made in the laboratory, during an industrial process, and in other environments as a means of measuring, controlling, or monitoring chemical processes and ionic impurities. Such measurements have been made for over 100 years. Measuring electrical conductivity is a sensitive means of monitoring processes that can be measured by instrumentation capable of detecting from the ng/L to kg/L concentrations of ions.
Ions are chemical species which carry a net positive or negative charge and are, by definition, conductive. Though conductivity measurements are proportional to ionic concentration, conductivity measurements cannot distinguish the specific chemical concentration since all ions have different ion mobilities (ability or efficiency with respect to carrying a charge). However, for well understood applications with known chemicals, determining conductivity is an excellent analytical industrial tool for measuring ionic concentrations. In other applications such as high purity water treatment systems found in micro-electronics, life sciences, and power generation applications, conductivity is an excellent analytical tool for measuring ionic impurities.
A typical analog conductivity measurement system consists of a sensor in direct contact with a fluid to be measured. A separate micro-controlled transmitter that typically contains AC measurement electronics, a readout or other visual display, software menus for instrument control functions, and other means to communicate to external devices such as a programmable logic controller (“PLC”) or data acquisition system used for industrial control systems is also typically present in such a conductivity measurement system. A cable, usually with a quick disconnect, is normally used to connect the transmitter to the sensor.
Therefore, a typical analog conductivity measurement system consists of a sensor, a transmitter, and a means of connection. Analog electrical conductivity (or resistivity=1/conductivity) measurement technology involves the use of an AC resistance measurement circuit connected to 2 (or more) electrically conducting electrodes. The electrodes are assembled in a mechanically rigid and unchanging geometrical configuration, separated by an electrically insulating material. The separation of the electrodes and the area of the electrodes in a fixed geometry are known traditionally as the sensor's “cell constant” (“cc”). To measure the conductivity of a fluid, the sensor is immersed in the fluid and the resistance of the fluid residing between the electrodes is measured by the AC resistance measurement circuit.
The resistance of the fluid between the electrodes is directly proportional to the distance between the electrodes. Therefore, resistance measurements need to be adjusted or normalized for the distance between the electrodes. Likewise the resistance of the fluid between the electrodes is inversely proportional to the surface area of the electrodes. Thus, the resistance measurement needs to be adjusted or normalized for the surface area of the electrodes. As a result, the measured resistance is adjusted, or normalized, for the cell constant, such that the cell constant=separation of the electrodes/surface area of the electrodes according to:Resistivity=Resistance/cell constantandConductivity=1/ResistivityIt is conductivity that is proportional to the ionic concentration. The cell constant may be determined according to the other conventional procedures.
For analog sensors, a complete calibration of the measuring system is required to meet certain regulatory standards and/or good calibration practices. For analog sensors, the standard calibration practice is to disconnect the sensor from the measurement electronics, attach a resistor having a known value to the measurement electronics, and adjust the measurement electronics as necessary. Verification of analog sensors may also be conducted in a similar manner. For analog sensors, the sensor is disconnected from the measurement electronics and a resistor having a know value is attached to the measurement electronics. The output of the measurement electronics is compared to the resistance value of the resistor having a known value. If the comparison is within an acceptable limit, no calibration is needed.
This calibration technique of the measurement electronics is possible because the AC measurement electronics are typically located in the transmitter of analog conductivity measuring systems. This procedure may need to be repeated if a multiple-point calibration function is employed to calibrate a single circuit. This process may again need to be repeated if there are, as is usually the case, multiple AC resistance measurement sub-circuits embedded in the design. These sub-circuits may be utilized to provide multiple measurement ranges. Once the measurement circuit has been verified and/or adjusted, the traceable resistor is disconnected, and the sensor is reattached to the measurement electrodes. As used herein the term “traceable resistor” refers to a resistor having a known value.
The next step in the calibration process is to verify and/or adjust the cell constant of the sensor. To do this, the sensor is immersed in a solution of known and traceable conductivity, and then the cell constant of the sensor is calculated according to various computations:New cc=old cc×(reference conductivity)/(measured conductivity)orNew cc=(measured resistance)/(reference resistivity)orNew cc=(measured resistance)*(reference conductivity)In general, the calibration of the complete analog conductivity measurement system (both the measurement circuit and the cell constant), is then complete.
Digital conductivity sensors may also be used to measure conductivity. Digital conductivity sensors are functionally equivalent to analog conductivity sensors, but with multiple advantages. In analog conductivity sensors, the analog AC resistance signals are easily degraded by the length of the cable, external noise, and other means unless extraordinary precautions are taken. In contrast, the transmitter of a digital conductivity sensor has all the features of an analog conductivity transmitter, except the AC measurement circuit is directly attached to the sensor electrodes. A cable is still used to attach the transmitter to the sensor. However, the small separation (e.g., <1 inch) between the electrodes and the AC measurement system of the digital conductivity measuring system provides significant inherent measurement advantages, as well as better performance in terms of measurement range and accuracy in comparison to analog conductivity measuring systems. The AC measurement circuit of the digital conductivity measuring system converts all the analog signals to a digital signal, which can be transmitted by the cable to the transmitter over long distances without any signal degradation.
One of the challenges of the digital conductivity measuring system is that direct integration of the AC measurement circuit to the sensor prohibits subsequent disconnection of the measurement circuit from the sensor. More specifically, this integration of the measuring circuit and the sensor prevents calibration of the measuring circuit in the manner associated with calibration of an analog conductivity measuring system.